N. Kumar, H. Rajashekhar, D. Vrushabendrakumar, K.M. Alam, S. Chitti, S.S. Aare, K. Shankar
University of Alberta,
Canada
Keywords: artificial photosynthesis, CO2 photoreduction, carrier dynamics characterization, plasmonic catalysis, green hydrogen production
Summary:
Noble metal nanoparticles have a long history of use in heterogeneous catalysis to reduce the activation energy of various thermally driven chemical reactions through efficient coupling of the phonon modes of the nanoparticles to the vibrational modes of reactant molecules. Our recent innovation consists of the formation of porous, sponge-like nanoparticles made of gold and gold alloys through a scalable solution-based process involving the etching of a sacrificial metal (silver). We have found that Au, AuPt and AuPd nanoponges are capable of catalyzing carrier-driven chemical reactions in addition to promoting phonon-driven chemical reactions, a development that has immense significance for chemical reaction engineering. The porosity of the sponge-shaped nanoparticles ensures a much larger surface area for reactant adsorption compared to traditional nanoparticle morphologies such as nanospheres, nanorods and nanocubes as well as a high density of low coordinate reaction sites such as steps and kinks. While the pore size in monometallic Au nanosponges was approximately 8-10 nm, the pore size in bimetallic AuPt nanosponges was found to be 3-4 nm. Conventional spherical Au nanoparticles absorb ultraviolet photons and exhibit a sharp surface plasmon resonance in the green spectral region in air and water, which results in the production of highly energetic hot carriers through interband damping and Landau (intraband) damping mechanisms. These hot electrons (and hot holes) are able to drive carrier-driven chemical reactions in the vapor phase or in liquid electrolytes by directly reducing (and oxidizing) adsorbed reactants. In Au nanosponges, the localized surface plasmon resonance is strongly red-shifted (to near-infrared wavelengths) and massively broadened. Thus photoexcitation of the nanosponges by blue and ultraviolet photons can be be used to directly trigger carrier-driven chemical reactions while infrared photons can be used to deliver thermal energy to the nanosponges for phonon-driven chemical reactions. When irradiated with 785 nm infrared photons, plasmonic Au nanosponges showed dramatically higher catalytic activity than hemispherical Au nanoislands for the transformation of para-aminothiophenol (PATP) into p,p'-dimercaptoazobenzene (DMAB). AuPt nanosponges were able to photoreduce gas phase CO2 to CO with 100% selectivity under AM1.5G one sun illumination at 70 deg.C in the presence of steam (H2O).